Cardiac rupture is a major lethal complication of acute myocardial infarction (MI). Despite significant advances in reperfusion strategies, mortality from cardiac rupture remains high. Both clinical and experimental studies have provided strong evidence that wall rupture at an early stage of MI is mainly the consequence of excessive myocyte loss and regional inflammation. Thrombolytic therapy aiming to reperfuse the ischemic myocardium showed conflicting results as they seem to increase the risk of aneurysm formation and rupture. The molecular mechanisms by which thrombolytic therapy affects cardiac remodeling and rupture are still largely unknown. Recent evidence has shown that the physiological functions of thrombolytic serine proteases extend beyond blood coagulation and play a pivotal role in modulating inflammatory and repair responses to tissue injury in part via protease-activated receptors (PARs). PAR1 is the predominant receptor for thrombin actions in cardiac cells and platelets. However, the use of PAR1 inhibitors to suppress remodeling was associated with severe bleeding, which limit their clinical use. We recently reported the expression of an additional thrombin receptor, PAR4, in the heart that is activated by high concentrations of thrombin, making PAR4 a potential therapeutic target in situations associated with high thrombin concentrations such as MI. However, little is known about PAR4 function in the heart. Our preliminary data in PAR4-deficient mice show that at 2 days after MI there is decreased myocyte death, reduced infarct size and improved functional recovery relative to wild-type (WT). However, at longer times after MI, PAR4-deficient mice showed impaired cardiac function, delayed infiltration of inflammatory cells, and greater rates of myocardial rupture relative to WT. Subsequent studies to delineate the mechanisms involved support a role of PAR4 signaling in neutrophil apoptosis, which plays an important role in inflammation resolution. These studies suggest that loss of PAR4 signaling in myocytes might be cardioprotective, explaining the early effects, but loss of PAR4 in inflammatory cells disrupts post-MI wound healing and can lead to cardiac dysfunction and wall rupture. We will investigate in aim 1 if activation of PAR4 signaling in cardiomyocytes after MI promotes myocyte death and cardiac dysfunction. In aim 2, we will determine if activation of PAR4 signaling in neutrophils after MI promotes their death after infiltration into the damaged heart to produce normal wound healing. Finally, aim 3 will investigate the impact of PAR4-mediated neutrophil apoptosis on inflammation resolution and cardiac healing post-MI. The proposed experiments will identify novel cell targets by which thrombolytic serine proteases and PAR4 influence myocardial healing post-MI independently of their effects on blood coagulation and will test whether targeting PAR4 in cardiomyocytes or inflammatory cells would offer novel strategies to reduce the incidence of mortality after MI, including mortality caused by ventricular rupture.